Varifocal objectives with two relatively axially shiftable components are well known in the art, e.g. from my prior U.S. Pat. No. 3,165,044. These components have coaxial lens mounts surrounded by a rotatable cylinder with camming grooves engaged by projections on the lens mounts whereby rotation of the cylinder about the objective axis moves each lens mount independently of the other at a rate determined by the pitch of the corresponding groove. Such camming grooves, which may be of linear or nonlinear pitch, are difficult to machine in a solid cylindrical tube or sleeve. The alternative technique of casting or molding the tube entails some difficulties in extracting the tube from the mold and also is less accurate as concerns the resulting cam shape. Similar considerations apply to the setting rings of iris diaphragms having grooves for the guidance of studs projecting from the iris leaves.